Sound muffling device



Dec. "7, 1937.

U. SCHMIDT SOUND MUFFLING DEVICE Filed April 25, 1935 INVENTOR.

mm m wfl m c% u Patented Dec. 7, 1937 t PATENT OFFICE 2,101,460 SOUND-MUFFLING DEVICE Ulrich Schmidt, Berlin wilmersdorf,

Application April 25, 1935, Serial No. 18,083 In Germany December 17, 1932 ficlaims.

My invention relates to sound muilling (18-.

vices, more particularly to devices for attenuating or silencing objectionable noise incident to the exhausts of internal combustion engines, intake noises in motors, objectionable noise produced by fans, and other noise generating appliances.

The known type of sound muiller heretofore used in the art especially in connection with internal combustion engines for deadening the exhaust noise usually comprises a main channel or conduit passing the exhaust gases or other sound carrying medium and a sound deafening or acoustic absorbent material suitably associated therewith serving to attenuate or disipate the pressure or sound waves carried by the stream of gas or other medium.

According to one construction of a sound muiller heretofore known in the art, the acoustic absorbent material consisting for instance of glass wool and the like is so disposed within the conduit passing the noise carrying gases, such as the exhaust gases in the case of an internal combustion engine, as toobstruct the flow of the gas or other sound carrying medium. According to a further construction as known in the art, the

absorbent material is arranged contiguous to the perforated main channel or conduit passing the exhaust gases 'or other sound carrying medium..

All these constructions have the disadvantage that due to the presence of foreign matter in the gas stream or other medium, such as soot or oil particles or other combustion residues in the case of internal combustion engines as combustion water, the interstices or pores of the sound absorbent material become clogged or encrusted within a short period, thus greatly impairing the in connection with whichthe sound murder is used, such as an internal combustion engine or the like.

In accordance with a further improvement, the

acoustic absorbent material is disposed in such a manner relative to the perforated main channel or conduit passing the exhaust gases or other sound carrying material as to leave a free space in such a manner that the exhaust gases would not be allowed to come in direct contact with the sound attenuating or dissipating substance. 5 However, devices of this type have not been successful due to the presence of a difference in pressure between the points of ingress and egress of the exhaust gases passing through the main channel or conduit, whereby a major amount of 10 the gas current is shunted into the lateral or intermediary space, thus passing closely to the sound absorbing material and causing a clogging or encrusting in the same manner as in the previously described arrangements. In addition, 15 this construction has the disadvantage that only a limited amount of sound absorbing material may be provided, thus impairing the efllciency of the sound muiiler as it is well known that the efilciency is proportional to the amount of absorbent material used.

Accordingly the object of my invention is to provide a sound muiiler of thecharacter described which is simple and easy in construction and highly efllcient in silencing or attenuating undesired pressure or sound waves superimposed upon a noise carrying medium such as the stream of exhaust gases in an internal combustion engine.

'A further object of my invention is the provision of a sound muflling device in which the 30 sound dissipating or attenuating material is disposed in such a manner as to be substantially free from interference by' the sound carrying medium such as the exhaust gases in internal combustion engines.

A further object of my invention is the provision of a sound muiliing device for dissipating or attenuating objectionable sounds or noises carried by a moving medium such as the exhaust gases in an internal combustion engine whereby the acoustic absorbent material is acted upon by the sound waves to be dissipated only and is substantially free from interference by any foreign matter or additional constituents contained in the sound carrying medium.

Another object of my invention is the provision of a selective sound muiller which is simple in construction and allows of an easy adjustment of the frequency or band of frequencies of the sound or pressure waves to be dissipated or attenuated.

Still a further object of my invention is the provision of a sound muilling device in the form of asound filter which allows of an easy adjustment and design for dissipating or attenuatable sound waves or noises which it is desired to suppress.

The invention has other objects in view which will appear hereafter in the detailed description thereof in connection with the accompanying drawing in which I have shown a few exemplifications of a sound muflier specially adapted for suppressing the exhaust noise in internal combustion engines.

Figure 1 shows an elevation view with parts broken away of one form of embodiment of a i sound miifiier according to the invention.

Figure 2 shows a modified form of a .muflier according to the invention; and

Figure 3 shows an'embodiment of a sound muflier according to the invention for securing a filter efiect for attenuating or dissipating a broad band of sound frequencies.

Similar reference numbers identify similar parts throughout the different views of the drawpreferably a right angle with the direction of flow of the sound carrying medium or the main channel or conduit through which the sound carrying medium such as the exhaust gases of an internal combustion engine is passed. The invention is further differentiated from the prior art devices in that the acoustic absorbent material is arranged or packed in a chamber disposed laterally to and acoustically coupled with the main channel or conduit through which the sound carrying medium is passed in such a manner that the exhaust gas current or other sound carrying medium does not come in direct contact with the sound absorbing material, thus preventing any clogging or encrusting of the latter. The exhaustgases directly pass through the smooth main conduit while only the sound waves due to their tendency to radiate in all directions are absorbed by the noise deadening lateral chamber forming a reactive side branch or sound trap to use the analogy of wave traps in electrical circuits.

As is understood, in order to prevent the exhaust gases or other sound carrying medium from entering the soundabsorbing chamber, no substantial physical coupling-of the latter should exist with the main conduit with regard to the 1 gas stream passing through the main conduit to use a further analogy of electrical circuits. In other words, no substantial pressure difference between separate points of the exhaust gas stream should be applied at separate ingress and egress points of the sound chamber which is likely to result in a substantial shunt or bypassing stream of the exhaust gas or other medium through the sound chamber and the absorbing material resulting in the drawbacks and disadvantages pointed out above. According to the present invention, this object is obtained by the provision of a closed imperforate sound absorbing chamber or chambers coupled substantially acoustically only with the main channel or conduit through which the sound carrying medium is passed in such a manner that only the sound or pressure variations carried by the latter are allowed to readily enter the lateral absorbing chamber, which latter is substantially suppressed by the muflier.

'ing a band of predetermined width of objectionphysically decoupled from themain channel or conduit with regard to the stream of the exhaust gases or other medium passed therethrough. I In the example illustrated, the latter object and effect is secured by the provision of .a single narrow connecting path between the main conduit and an absorbing chamber sumcient to provide eflicient acoustic coupling for the pressure variations 'of a predetermined frequency range to be eliminated, but insuficient to apply substantial pressure difference to the side chamber likely to result in a by-passing or shunting of sound carrying medium through the side chamber.

Extensive experiments have proven that a sound mufiier according to the invention possesses a considerably higher efiiciency as compared with sound mufliers heretofore known while at the same time presenting the advantage that the sound deadening material is free from becoming clogged or encrusted or otherwise being interfered with by the exhaust gases or other sound carrying medium and consequently from losing its acoustic or sound deafening effectiveness, In addition, the efficiency of the device may be further increased by an increase of the effective dissipating or sound dampening surface of the absorbent material by the use of a desired number of serially arranged lateral chambers or reactive side brancheseach disposed in shunt relationship to the main channel or conduit carrying the exhaust gases or other noise carrying medium.

Referring more particularly to Figure 1 of the drawing illustrating a simple form of embodiment of a sound mufiier especially adapted for use in connection'with internal combustion engines, I have shown at i a main duct or channel through which the exhaust gases or other sound carrying medium is passed. The channel i has an opening 2 connecting it with a closed laterally disposed tubular shaped chamber 3. Item 4 represents a foraminous or perforated tube leading from the opening 2 into the lateral chamber 3 as shown. The space between the tube 3 and the outer walls of the chamber 3 is packed with acoustic absorbent material shown at 5.

In a device of this type, as is understood, the

exhaust gases passing through the tube I are pre-' tering into the chamber 3 through the foraminous tube 5 are spread over the openings of the latter and impinged upon the absorbing material 5 in such a manner as to become attenuatedor dissipated in a known manner. Since the exhaust current does not come in contact with the tube 4, the latter may have a large surface and may be roughened to a desired degree without increasing L the resistance against the flow of the exhaust The sound waves enmain conduit I and extending radially from the latter.

Referring to Figure 2, this shows a modified construction of a sound muiiler according to the invention in which the lateral chamber encloses an annular space 6 disposed around the main conduit I. In this case the total sound dampening eifect may be increased by using a plurality of chambers 6 arranged around the main conduit I. In a construction of this type the main conduit I is interrupted for a length equal to the free annular space 1, thus allowing the sound waves at this point to radiate freely and in all directions into the annular space I while the gas current is passed through the conduit I without entering the annular side chamber. The latter is bounded by two wire sieves or foraminous metal sheets or discs as shown at 8 and the sound absorbent material 5 packed therebetween, as shown, and the outer walls of the chamber 9.

Since the shape of the chambers 3 or 6 in which the sound is dampened or deadened does not in any way interfere with the-exhaust current in the main conduit I, a device of the type according to the invention lends itself in an easy and simple manner to the construction of an acoustic filter by the use of a plurality of chambers or reactive side branches 3 and 5 of suitable diameter and length and properly spaced from each other. The acoustic filter functions in that those frequencies, such as the low notes not affected by the damping material by reason of the unitary construction of the lateral chambers will be filtered and deadened by the subsequent chambers.

Referring to Figure 3, this shows one embodi- 'ment of an acoustic filter according to. the invention comprising five chambers or reactive side branches III to I4 arranged in succession and constructed or tuned to be effective in suppressing a series of separate frequencies whereby each of these chambers will have a different selective effeet in dampening or dissipating the sound waves.

In the embodiment shown, the first chamber III is constructed in the form of a pure absorption chamber in such a manner that the cross section I5 of the sound channel enclosed by the sound dampening material 5 corresponds to the cross section I6 oi the coupling opening for the sound waves entering into the'chamber III. The next chamber Ii is designed in such a manner that the space enclosed by theopen annular chamber I1 on the one hand is larger than the space of the chamber I0, and on the other hand that the opening cross section I 8 for the sound waves from the main conduit I into the chamber H is lem than the open cross section I! of the latter. The relationshipv of the volume of the annular chamber II to the opening cross section may be given such a value that this arrangement by its filter action becomes effective in suppressing a predetermined low frequency band. The next chamher I 2 differs from the chamber II merely by the subdivision of; the opening cross section into several units I9 whereby -the acoustic filter eflect may be modified as compared to the chamber I I.

The construction of the next following chamber I3 is such that the glass wool or other absorbent material 5 forms a pair of discs 20 bounded by the perforated or foraminous sheets Sand 8 and disposed at a predetermined distance from the walls of the' chamber 2|. In an arrangement of this nature the sound waves penetrate the glass wool or other absorbent material 5 in such a manner as to enter freely into the space 22 behind the absorbent material 5 to be impinged upon the walls will be understood, results in an increased dampening or dissipating efiect and the attainment of a definite selectivity determined by the distance between the dampening surfaces from the walls of the chamber 2L. The last chamber I4 is a modification of the chamber I3 whereby in order to prevent standing waves in the air space 23 the entire chamber I4 with the exception of the annular slot bounded by the coupling opening 24 is packed with acoustic absorbent material in a manner similar to the first chamber Ill.

The separate chambers ID to I4 may be designed individually in accordance with the constructions shown by Figures 1 and 2 or they may form a unitary structure as shown in Figure 3. In the latter, the walls of one chamber form the boundary for the next following chamber whereby the entire structure forms a simple compact unit. In place of the perforated sheets 8, wire sieves or the like means may be provided for allowing the passage of the sound waves, and

tubular chambers as shown in Figure 1 may be substituted for the annular chambers illustrated in Figure 3, as will be readily understood.

As is obvious fromthe above, a sound muiiiing device according to the invention has many additional uses for suppressing or silencing objectionable noises such as the intake noise in motors, noise produced by fans, firearms, etc. and in general appliances operating with a noise producing moving'medium.

While I have shown the invention embodied in the specific constructions exemplified in the drawing, it will be obvious that the same is susceptible of various modifications andvariations coming within its broader spirit and scope as expressed by the ensuing claims.

I claim:

i. A sound mufiler comprising a conduit providing a substantially unobstructed gas flow and having a circular opening, a closed imperforate annular chamber surrounding said conduit, a plurality of perforated partition walls arranged substantially transversely to said conduit and dividing said chamber into a number of adjacent compartments, and packings of sound absorbing material filling alternate compartments, the central non-filled compartment having a width equal to said opening and communicating therethrough with said conduit to provide an acoustic coupling path therebetween and said conduit.

2. A sound muiiier comprising a conduit providing a substantially unobstructed gas fiow, a plurality of side chambers coupled to said conduit acoustically only so as not to form part of the to the gas fiow through said conduit and dividing each of said chambers into two compartments, one compartment of each of said chambers communicating with said conduit through annular openings therein adapted to provide acoustic coupling paths but forming no part of the path traversed by the gas, and packings of sound absorbing material filling the other com- 'partments of each of said side chambers.

4. A sound muifler as claimed in claim 2 in which said side chambers are of diflerent size so that each is efiective in dissipating sounds of a different frequency range. 2

5. A sound muffler comprising a conduit'providing a substantially unobstructed gas flow and having an opening therein, a closed imperiorate annular chamber surrounding said conduit, and a plurality of perforated partition walls arranged transverse to said conduit and dividing said chamber into a number of adjoining compartments, packings of sound absorbing material fill- ULRICH scrnum'r. 

